Field of the Invention
The invention concerns a near field reflection microscopy process and microscope using a waveguide of the optic fibre type as a probe of this field.
Traditional near field microscopy takes advantage of the particular structure of an electromagnetic wave in the neighborhood of a diameter opening smaller (and up to ten times larger)than its wavelength. The neighborhood should here be understood as being the area called "near field" which is situated at a distance from the opening smaller than the wave length. In this area, the wave intensity rapidly decreases until it reaches the value of the intensity within the area called "far field" which can be calculated by a known way, by application of Maxwell equations.
This extremely rapid decrease of the intensity depending on the distance from the opening enables to survey with accuracy the topography of the surface from where the light is emitted, or re-emitted, each variation of intensity measured by appropriate means corresponding to a distance variation.
A number of devices are known which operate according to this principle, both in transmission and in reflection. For example, European patent EP-112 401, filed 27th Dec. 1982 by International Business Machines Corporation, is directed to overcoming the problem of obtaining a small diameter opening, typically smaller than the micrometer, usable in an optical near field microscope. In an embodiment described in this document, the opening receives the light transmitted through a sample to be analyzed, with the opening being kept at an appropriate distance by a traditional means such as an electronic probe measuring tunneling current of electrons.
The use of such an electronic probe nevertheless makes it necessary to metallize the sample to be analyzed, the submicronic opening being already metallized, since it is limited by a thick layer of metal, opaque to the measuring electromagnetic wave.
This double metallization of the sample and the optical probe composed by the submicronic opening, has serious inconveniences.
Indeed, the metallization of organic samples, or more generally dielectrical, may form an obstacle for the observation of certain phenomenae, namely dynamic or living phenomenae. Moreover, this metallization may be heterogeneous or hide small scale structures by overlapping. In addition, in the case of discontinuity of the superficial conductor layer, the control of the distance to the opening is no longer assured, which leads to a topographic aberration.
Finally, it was noted that the metallic walls of the submicronic opening and the metallized surface of the sample make a resonant cavity causing interferences which modulate the intensity measured behind this opening and perturbs the measure.
In addition, the opening of a submicronic diameter is the main difficulty in realizing this kind of microscope, and limits its development. Indeed, the repetitive manufacture of a submicronic opening, especially for a near field transmission microscope is particularly difficult to put into operation and is the object of a solution described in European patent EP112 402, filed on 27th Dec. 1982 by International Business Machines Corporation. The diameters of the openings realized by the process described in this document vary between 10 to 500 nanometers. The horizontal resolution of a near field microscope equipped with such an opening then depends directly on its diameter and on its distance from the sample--the resolution is in the region of the greater of these two quantities.
The same difficulties of implementation exist for the near field reflection microscope. For this type of microscope, specific optical probes have been developed in emission and reception, and namely that described in European patent EP185 782, filed 28th Dec. 1984 in the name of International Business Machines Corporation. In this document, is described a compound wave guide, the terminal section of which is annular, so as to present:
a first submicronic opening, delimited by an opaque metal layer operating in emission,
and a transparent peripheral ring, delimited by the first opaque layer and a second opaque metal layer operating in reception.
The measurement of the reflected light is carried out in this way through a planar peripheral waveguide composed by the transparent peripheral ring; this configuration is unfavorable for a reliable measurement insofar as such a planar structure will inevitably introduce undesirable electromagnetic propagation modes.
Finally, other known microscopic techniques, and in particular optical microscopy by phase contrast--such as described for example in an article published in the magazine Electronics Letters, vol. 22, No. 2, January, 1986, pages 103-105, called "Fibre-optic scanning differential interference contrast optical microscope", written by Mr. VAEZ IRAVANI,--or again the scanning optical microscopy--such as described in the application for a European patent EP-283 256 filed 16th Mar. 1988 in the name of TEKTRONIX Inc, do not use the near field phenomenon and are consequently limited in resolution by the far field diffraction phenomenae.